Why Do We Have Different Eye Colours?

The colour of the human iris depends primarily on the concentration and distribution of melanin pigment in the stroma (front layer) of the iris. Brown eyes have a high concentration of melanin, which absorbs most light. Blue eyes have very little melanin; the colour comes from Rayleigh scattering of light in the stroma (the same principle that makes the sky blue). Green and hazel eyes have intermediate amounts. At least 16 genes influence eye colour, with OCA2 and HERC2 on chromosome 15 playing the largest roles. This polygenic inheritance explains why two blue-eyed parents can occasionally have a brown-eyed child.